A metal oxide semiconductor field-effect transistor (MOSFET) has been the building block for most computing devices for the last several decades. A MOSFET is a four terminal device made up of a drain, a source, a gate and a bulk substrate. In digital circuits, the MOSFET is essentially used as a switch. The source and drain are two ends of the switch, with the channel being turned on and off under the control of the gate. The gate controls the conductivity through the channel through an electric field and is insulated from the channel by a thin layer of dielectric material, such as silicon dioxide. With no voltage potential across the gate and bulk, a depletion region forms around the drain and source blocking any current flow.
The MOSFET has been the primary building block of integrated circuits for more than forty years. The advances in electronics have been driven primarily by the ability to scale down the size of the MOSFETs used in integrated circuits. This scaling achieves improvements on many fronts. Smaller transistors allow more transistors to be put on the same size chip, which has allowed integrations levels to rise from the hundreds of transistors to hundreds of millions of transistors.
Shrinking the feature size of the transistor also makes each transistor work faster and consume less power (this should not be confused with lower chip power, since the number of transistors per chip generally increases faster than power consumption per transistor decreases). The increase in speed comes from two factors, decreased capacitance and increased current. The capacitance of wires and gates decreases as the wire and gate elements decrease in size, thus, the amount of charge a transistor has to place on a wire or gate decreases.
A multi-gate device or multiple gate field-effect transistor (MuGFET) refers to a MOSFET that incorporates more than one gate into a single device. The multiple gates may be controlled by a single gate electrode, wherein the multiple gate surfaces act electrically as a single gate, or by independent gate electrodes. A multi-gate device employing independent gate electrodes is sometimes called a Multiple Independent Gate Field Effect Transistor (MIGFET). Multi-gate transistors are one of several strategies being developed by Complementary metal-oxide-semiconductor (CMOS) semiconductor manufacturers to create ever-smaller microprocessors and memory cells.